Sure it's possible. What will happen though is that the CC will work its way to the top eventually. There is no advantage to doing it that way. The worms, copepods, and other critters you are seeking to cultivate in a DSB will not do well in CC because it's too large a grain size. Also, the large grains allow more oxygen to penetrate the sand bed which inhibits the anoxic bacteria that you're trying to grow. It's best to go with one or the other. If you have CC, you'll need to vacuum it to control detritus. Sand is a real pain to vacuum! If you go with the DSB, you'll want it to remain undisturbed except for the critters that stir it. Either way will work...it really depends on what kind of tank you want and what you want to keep in it. If you can tell us more about what you want to keep, we may be able to make some suggestions that will save money and headaches down the road.